1. Field of the Invention
The present invention relates to a power output apparatus and a method of regulating power. More specifically, the invention pertains to a power output apparatus for outputting power from an engine to a drive shaft with a high efficiency as well as to a method of regulating the power output to the drive shaft.
2. Description of the Related Art
In known power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically linked with a drive shaft, which connects with a rotor of a motor, by means of an electromagnetic coupling, so that power of the engine is output to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). In this power output apparatus, the vehicle starts driving by the function of the motor. When the revolving speed of the motor reaches a predetermined level, the power output apparatus supplies an exciting electric current to the electromagnetic coupling to crank the engine and carries out supply of a fuel into the engine and ignition with a spark, thereby starting the engine. After the engine starts, the power from the engine is output to the drive shaft via the electromagnetic coupling, in order to continue driving the vehicle. When the power output to the drive shaft via the electromagnetic coupling is insufficient for the required power on the drive shaft, the motor is driven to supplement the insufficiency. The electromagnetic coupling regenerates electric power according to a slip of its electromagnetic connection while the power is output to the drive shaft. The regenerative electric power is stored in a battery as electric power used for starting the vehicle or is used as a power of the motor to supplement the insufficiency of the power on the drive shaft.
The conventional power output apparatus, however, has a problem of reduced efficiency with an increase in revolving speed of the drive shaft. In the power output apparatus discussed above, in order to output the power to the drive shaft via the electromagnetic coupling when the revolving speed of the drive shaft increases, it is required to make the revolving speed of the engine equal to or higher than the revolving speed of the drive shaft. An efficient driving range of the engine is generally defined by the revolving speed and the loading torque thereof. When the drive shaft rotates at a revolving speed exceeding the efficient driving range, the engine is forced to be driven out of this efficient driving range. This undesirably lowers the efficiency of the whole power output apparatus.